My god, some of the reactions on this thread..
Look. It's obviously not acceptable behaviour. Perhaps he should know better. BUT. He's 9 years old, willies and bums are funny at that age, not sexual. It's not like it's the teacher pulling his pants down.
Yes, the child in question should be spoken to and it should be explained that it isn't acceptable behaviour. He should be given an age-appropriate explanation about why this is the case.
At the moment in which he did it, he will have done so for a laugh. Okay, so it's bad judgement. I remember the lads in my school when I was that age, and they used to moon at the girls all the time, and we'd tell the teachers. They didn't turn into rapists (actually one did, but that's beside the point..)
But a child of that age, with a mostly innocent knowledge of the world around him, would have little to no idea of how important it is to keep sexual organs private, or have a concept or abuse or rape or anything like that. And in a way, that's good. Let the childlike innocence go on for as long as possible. You're all only having this reaction because you know the real world in much darker detail.
This child is not a sexual predator.
For goodness sake. Bullying? Police? Expulsion? Sexual assault?
Blimey. I'd hate to be in court if you lot were on the jury.